Leaving aside the very expensive truly limited editions, and the original 1st/1st publications, these are the ones I own and love most of all.
Best single volume:
Deluxe Slip-cased Edition - Allen & Unwin, 1969, ISBN 004823091X
Deluxe Slip-cased Edition (quarter-bound leather) - Harper Collins, 1997, ISBN 0261103687
Expect to pay 500 pounds or more for a 1st edition Deluxe Allen & Unwin, around 300 for a Harper Collins (less for later printings). But they are both truly lovely books - much better quality than the Harper Collins 50th Anniversary Deluxe Edition, 2004, ISBN 0007182368. In fact that whole run of The Lord of the Rings, The Hobbit, The Silmarillion & The Children of Hurin (previously mentioned in this thread) lack much of the quality of earlier deluxe editions.
Best three volume edition:
Hardback Illustrated Centenary Boxset - Harper Collins, 1992, ISBN 0261102958
Much easier to handle and read than the unwieldy single volume (or the equally large 3 volume movie tie-in 2008 release) but almost impossible to find as a boxed set of three, though it is sometimes possible to find individual hardback volumes of this edition. I think I paid around 300 pounds for mine, but I have seen them as high as 600 pounds. A lovely and very collectible set. The deluxe version (just 250 copies, ISBN 0261102990) usually goes for around 1000 pounds.
Best seven volume edition:
Hardback Millennium Edition Boxset (+ CD) - Harper Collins, 1999, ISBN 0261103873
The easiest to read (nice & small) and in six volumes as Tolkien intended (seventh is Appendices). Becoming increasingly hard to find in hardback, but still available for around 100 pounds (a lot less for the paperback version). These are the ones I reach for most of all when I fancy a read.
Sean.
ISBN numbers from
https://www.tolkienbooks.net/ not always reliable.